This invention relates to power supplies for ultrasound radio frequency (RF) transmitters. In particular, this invention relates to power supplies providing at least two different levels of voltage and instantaneous (peak) power.
Diagnostic medical ultrasound imaging operates in both B-mode and continuous wave (CW) Doppler modes. Two-dimensional B-mode imaging typically uses short transmitted bursts of higher peak voltage and higher peak power than CW Doppler. For B-mode imaging, the transmit sequence along any given ultrasound line occurs over a short period of time, but may have a peak power of hundreds of watts. For CW Doppler mode imaging, the transmitter is substantially constantly transmitting with a lower voltage to prevent excess output power dissipation. The B-mode and CW Doppler mode transmissions are often interleaved. FIG. 1 shows transmit pulses and associated timing for B-mode and CW Doppler interleaved operation. The amplitude of the transmit signals for the B-mode imaging is greater than the amplitude of the CW Doppler transmit signal.
Different supply voltages may be provided by switches and multiple DC power supplies. The low voltage supply is generally fixed and the HV supply generally variable, but both can be varied, depending on the operation. For example, FIG. 8 shows a single pole switch 302 that connects a high voltage supply 308 to a diode 306 connected with a low voltage supply 310. When the switch 302 is open, the transmitter 300 draws current from the low voltage supply 310 through the diode 306. When the switch 302 is closed, the high voltage supply 308 supplies the current and the diode 306 is reverse biased. A bypass capacitor 314 at the transmitter 300 provides radio frequency bypass to ground and local energy storage to minimize interference to low-level circuitry.
High to low voltage transitions may require dissipation of excess energy stored in the bypass capacitor (e.g., dynamic power dissipation). The amount of energy dissipated is the difference between the high and low voltage squared multiplied by the capacitance, all divided by two.
To avoid such dissipation concerns, conventional ultrasound imaging transmitters use separate power stages for multiple switching schemes. For example, amplifier stages are used. The difference between the supply voltage and the instantaneous output voltage is applied across an amplifier""s output. For efficient operation, the amplifier is supplied with a low voltage for low output power and a high voltage only for high output power.
Other arrangements may be used. FIG. 2 illustrates a multiple power supply amplifier 100 as disclosed in U.S. Pat. No.3,961,280. The amplifier 100 includes an amplifying transistor 102 connected with a load 104, a switching transistor 106, a high voltage source 108, a low voltage source 110 and a diode 112. The amplifier 100 adjusts the power supply voltage in response to the magnitude of the input signal or the signal to be transmitted. When an input signal 114 is less than the voltage from the low voltage source 110, the diode 112 is forward biased, and the switching transistor 106 is turned off. The amplifying transistor 102 is powered by the low voltage source 110. When the input signal 114 is larger than the voltage provided by the low voltage source 110, the base-emitter junction of the switching transistor 106 becomes conductive, reversing the voltage applied to the diode 112. The switching transistor 106 conducts the higher voltage from the high voltage source 108 to the load.
As an alternative to detecting the instantaneous magnitude of the input signal for selecting power supplies described above, programmable amplifiers may be used. For example, U.S. Pat. No. 6,078,169 discloses a programmable power supply circuit. Analogous circuitry 200 is shown in FIG. 3. A plurality of switches 204a-d and power supplies 202a-d are provided for selecting and powering a transmit array 208. A micro-controller 206 controls the switches 204a-d to select one of the power sources 202a-d. Like FIG. 2, excess energy stored in the bypass capacitance of the transmit array is dissipated in a resistor or used to recharge a power source.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include ultrasound multi-level power supplies for ultrasound imaging.
In one embodiment, a coupling capacitor is provided indirectly between a high voltage source and a low voltage source. The coupling capacitor provides direct current restoration for a two level power supply selector connected with an ultrasound transmit array.
In a further embodiment, a clamping circuit, including a capacitor and a diode, is provided indirectly between the high voltage and the low voltage sources. The high voltage source connects as a power supply to a pulser. Control signals are provided to the pulser to switch the output between the high voltage and ground depending on the type of ultrasound signal being transmitted. A voltage rail of the transmit array connects with the clamp circuit and receives a low voltage for CW Doppler mode imaging and a higher voltage for B-mode imaging. This low cost circuitry may simplify and improve the delivery of high voltage in a two level power supply selection in ultrasound imaging.
In a first aspect, a two level power supply for ultrasound imaging is provided. A coupling capacitor is connected indirectly between first and second voltage supplies. The second voltage supply has a higher voltage than the first voltage supply. At least one ultrasound transmit element operatively connects with the first and second voltage supplies.
In a second aspect, another two-level power supply for ultrasound imaging is provided. A plurality of ultrasound transmit cells connects with a voltage rail. A capacitor-coupled clamp circuit also connects with the voltage rail. A bias port of the capacitor coupled clamp circuit connects with a first voltage supply. An input of the capacitor coupled clamp circuit connects with a pulser. A second voltage supply provides power to the pulser.
In a third aspect, a method for providing power in an ultrasound imaging system is provided. A higher voltage is applied to an ultrasound transmit element. A lower voltage is then applied to the ultrasound transmit element. During generation of the lower voltage, direct current is restored for subsequent repetition of the generation of the higher voltage. The lower voltage characterizes the level of restoration.
In a fourth aspect, a method for providing power in an ultrasound imaging system is provided. High and low voltages are supplied. A high transmit voltage is generated and comprises the sum of the high and low voltages. A low transmit voltage is generated comprising the low voltage. During generation of the high transmit voltage, a capacitor is discharged. The capacitor is recharged with the low voltage.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.